diff --git a/panda/src/pgraph/alphaTestAttrib.cxx b/panda/src/pgraph/alphaTestAttrib.cxx
index 08705b74b6..f3994fb14e 100644
--- a/panda/src/pgraph/alphaTestAttrib.cxx
+++ b/panda/src/pgraph/alphaTestAttrib.cxx
@@ -61,6 +61,7 @@ void AlphaTestAttrib::
 output(ostream &out) const {
   out << get_type() << ":";
   output_comparefunc(out,_mode);
+  out << "," << _reference_alpha;
 }
 
 ////////////////////////////////////////////////////////////////////